HONG KONG, Nov 10 (Reuters) - Spanish telecoms giant Telefonica SA is seeking to raise $859 million to $875 million by selling a stake in China Unicom (Hong Kong) Ltd , according to a term sheet seen by Reuters on Monday.

Telefonica is offering 597.8 million China Unicom shares for HK$11.14 to HK$11.34 each, or 1.2 percent to 3.0 percent less than the shares' last traded price, the term sheet showed.

It was unclear why Telefonica, which has been battling high debt, plans to cut its stake. Debt at the Spanish company rose by 1 billion euros ($1.25 billion) in the second quarter to 43.8 billion euros.

Telefonica is barred from selling any more shares for the next 90 days, the term sheet showed.

In June 2012, Telefonica sold half of its stake in China Unicom to raise $1.4 billion. Telefonica held about 1.19 billion China Unicom shares, or a stake of about 4.98 percent, prior to Monday's sell down, according to Thomson Reuters data.